One concern in vehicular transportation is the reduction of vibrations and isolation of the passenger compartment from such vibrations. Two particular systems to reduce such vibrations are automotive suspension systems and mountings for engines. A front end suspension system is made up of various arms, rods, links, etc. which are connected together by means such as a bushing assembly. Bushing assemblies function to dampen out vibrations of the suspension system, induced, for example, by road surfaces. Such vibrations may comprise a range of amplitudes and frequencies.
In many front wheel drive automobiles, the engine is mounted transversely in the engine compartment and is supported on mounts and connected to a transverse frame component of the automobile by means of a torque strut. The torque strut functions to control displacements of the engine as well as dampen out vibrations which vary over a wide range of amplitudes and frequencies.
Known suspension system components and torque struts generally include an elongate member having at least at one end a bushing assembly for receiving connecting bolts. In torque struts, such bushing assemblies are mounted at both ends of the strut. Rubber bushings comprising annular elongate inner and outer members with elastomer disposed therebetween are well-known and are used to control movement but have limited capability in damping vibrations of low or high frequencies. Damping of vibrations of low or high frequencies occurs when an offsetting vibratory motion is generated within the bushing assembly that acts to cancel out the undesired vibratory motion. Elastomer of differing hardness can be used to effect damping due to the hysteresis property of rubber. For example, high durometer elastomers can be used to offset large low frequency radial motions between the inner and outer members and low durometer or soft elastomers can be used to offset small high frequency radial motions between the inner and outer members of the bushing. However, in general, rubber bushings can be said to have a constant spring rate and provide little or no damping.
One type of bushing assembly to overcome these shortcomings and provide suitable damping for some frequencies of vibratory motion are fluid filled bushings. Fluid filled bushings generally include a cylindrical elongate inner rigid member, an elongate outer rigid sleeve member concentrically disposed and radially spaced from the inner member and a resilient means disposed between the inner member and outer sleeve member wherein the resilient means defines a pair of diametrically opposed fluid filled chambers fluidly connected by an elongate restricted passageway. In operation, in response to vibratory motions between the inner member and outer sleeve member, fluid is displaced from one chamber via the restricted passageway to the second chamber in a direction opposite to the vibratory motion. In particular, when a first chamber is contracted the fluid is displaced therefrom through the restricted passageway to an expanding second chamber. In the reverse cycle of the vibratory motion, when the first chamber is expanding and the second chamber is contracting the fluid is reversibly moved through the restricted passageway. As can be seen, an oscillatory motion of the fluid is generated within the restricted passageway. The restricted passageway has a certain volume confining the fluid, wherein there is a mass of fluid. The oscillatory fluid in the restricted passageway creates a mass like resistance to the pumping forces of the chambers resulting in damping of the vibratory motions. The sizing of the restricted passageway allows the bushing to be designed, or tuned to provide high damping or dynamic stiffness in the frequency range from about 150 to 200 Hertz. It can be said that such bushing assemblies provide for both controlled movement and damping of vibratory motions at high frequencies.
For a more general discussion of the operational characteristics of fluid filled isolators, reference is made to an article entitled A New Generation of Engine Mounts, by Marc Bernuchon, SAE Technical Paper Series 840259, 1984, the disclosure of which is incorporated by reference herein.
An example of a fluid filled bushing is disclosed in U.S. Pat. No. 3,642,268. The bushing there disclosed utilizes hydraulic fluid displaceable between two chambers via a restricted orifice. The chambers are located in the bushing along a first radial direction whereas along a second radial direction perpendicular to the first radial direction is a solid rubber member. Such a bushing exhibits high stiffness and low damping along the second radial direction and low stiffness and high damping along the first radial direction dependent on the flow characteristics between the chambers and the fluid properties as described heretofore. Other representative patents disclosing similar bushings include U.S. Pat. Nos. 3,698,703; 4,588,174; 4,605,207; 4,630,806; 4,667,942 and 4,377,216.
While the fluid filled bushings disclosed in the referenced patents function satisfactorily for their intended use at limited frequencies and amplitudes, there is a need for a fluid filled bushing assembly which can function satisfactorily over a range of frequencies and amplitudes of vibration. Certain applications require damping at low amplitude in the frequency range from about 5 to about 50 Hertz in addition to the damping for high frequencies. For example, isolation of engine idle oscillations is improved by using a mount of soft or low dynamic stiffness. However, for large road induced motions which result in large amplitude, low frequency vibrations, the damping available in the fluid filled bushing as described in the '268 patent is inadequate to provide damping in the frequency range from about 5 to about 50 Hertz.